Deforming is a technique used to increase the strength of a material through a precision cold reduction targeting a specific proof strength or tensile strength. The surface finishes for deformed stainless steels for instance by temper rolling are denoted according to the standard EN 10088-2 as 2H and according to the standard ASTM A666-03 as TR.
The standard austenitic stainless steels such as 301/EN 1.4310, 304/EN 1.4301 and 316L/EN 1.4404 are used in temper rolled condition performed for the purpose of strength adjustment. Thanks to work hardening a high strength is obtained. Further, due to hardening caused by strain induced martensitic transformation in deformed portions, the so-called TRIP (Transformation induced plasticity) effect, the steels 301 and 304 have excellent workability. However, a decrease in workability accompanying an increase in strength is unavoidable. This behaviour is applied in the U.S. Pat. No. 6,893,727 for a metal gasket manufacturing of an austenitic stainless steel containing in weight % at most 0.03% C, at most 1.0% Si, at most 2.0% Mn, 16.0-18.0% Cr, 6-8% Ni, at most 0.25% N, optionally at most 0.3% Nb, the rest being iron and inevitable impurities. The microstructure is advantageously either a dual phase structure having at least 40% martensite and the rest of austenite or a single phase structure of martensite.
The U.S. Pat. No. 6,282,933 relates to a method of manufacturing a metal carcass for use in a flexible tube or umbilical. The method contains a work-hardening step for the metal strip before shaping and before winding the strip to form a carcass. According to this patent all the metals which after work-hardening have a yield strength higher than 500 MPa and an elongation at rupture of at least 15% can be used to manufacture a metal carcass. However, this U.S. Pat. No. 6,282,933 also describes that it was already known that duplex and superduplex materials, used for the manufacture of metal carcasses, do not need to be work-hardened since they fulfill the above mentioned demands without work hardening. The work-hardening according to this U.S. Pat. No. 6,282,933 is done for austenitic stainless steels, for instance 301, 301 LN, 304L and 316L, in order to make possible to use these materials for the manufacture of metal carcasses.
The EP patent application 436032 relates to a method of producing high-strength stainless steel strip having a dual ferrite/martensite microstructure containing in weight % 0.01-0.15% carbon, 10-20% chromium and at least one of the elements nickel, manganese and copper in an amount of 0.1-4.0 for springs. For the dual ferrite/martensite microstructure the cold rolled strip is continuously passed through a continuous heat treatment furnace where the strip is heated to a temperature range for two-phase of ferrite and austenite and, thereafter the heated strip is rapidly cooled to provide a strip of a dual structure, consisting essentially of ferrite and martensite and, further, optionally temper rolling of the dual phase strip at a rolling degree of not more than 10%, and still a step of continuous aging of no longer than 10 min in which the strip of the dual phase is continuously passed through a continuous heat treatment furnace. Because the object of this EP 436032 is to manufacture a spring material, the spring value can be improved with temper rolling before aging.
The GB patent application 2481175 relates to a process for manufacturing a flexible tubular pipe using wires of austenitic ferritic stainless steel containing 21-25 weight % chromium, 1.5-7 weight % nickel and 0.1-0.3 weight % nitrogen. In the process after annealing at the temperature range of 1000-1300° C. and cooling, the wires are work-hardened by reducing the cross-section at least 35% so that the work-hardened wires have a tensile strength greater than 1300 MPa. Further, the work-hardened wires are wound up directly after the work-hardening step retaining their mechanical properties.
The object of the present patent application is to eliminate some drawbacks of the prior art and to achieve an improved method for producing high-strength ferritic austenitic duplex stainless steel with the attained TRIP (Transformation induced plasticity) effect by deforming in such a manner, that the retained formability at high strength level can be utilized in the ferritic austenitic duplex stainless steel.
In the method according to the present invention a ferritic austenitic duplex stainless steel with the attained TRIP (Transformation induced plasticity) effect is first heat treated at the temperature range of 950-1150° C. After cooling, in order to have high tensile strength level of at least 1000 MPa with retained formability the ferritic austenitic duplex stainless steel is deformed with a reduction degree of at least 10%, preferably at least 20%, having the elongation (A50) at least 15%. With the reduction degree of at least 40% the ferritic austenitic duplex stainless steel achieves the tensile strength level of at least 1300 MPa and has the elongation (A50) at least 4.5%. After deformation the ferritic austenitic stainless steel is advantageously heated at the temperature range of 100-450° C., preferably at the temperature range of 175-250° C. for a period of 1 second-20 minutes, preferably 5-15 minutes, to improve the strength further whilst retaining an elongation (A50) of at least 15%. In addition to the already well known high corrosion properties the deformed duplex stainless steel with the attained TRIP effect has improved strength to ductility ratio, the fatigue strength and the erosion resistance.
In one preferred embodiment (A) the duplex stainless steel with the TRIP effect in accordance with the invention contains in weight % less than 0.05% carbon (C), 0.2-0.7% silicon (Si), 2-5% manganese (Mn), 19-20.5% chromium (Cr), 0.8-1.5% nickel (Ni), less than 0.6% molybdenum (Mo), less than 1% copper (Cu), 0.16-0.26% nitrogen (N), the sum C+N being 0.2-0.29%, less than 0.010 weight %, preferably less than 0.005 weight % S, less than 0.040 weight % P so that the sum (S+P) is less than 0.04 weight %, and the total oxygen (O) below 100 ppm, optionally contains one or more added elements; 0-0.5% tungsten (W), 0-0.2% niobium (Nb), 0-0.1% titanium (Ti), 0-0.2% vanadium (V), 0-0.5% cobalt (Co), 0-50 ppm boron (B), and 0-0.04% aluminium (Al), the balance being iron (Fe) and inevitable impurities occurring in stainless steels. This duplex stainless steel is known from the WO patent application 2012/143610.
The duplex stainless steel of the embodiment (A) has the yield strength Rp0,2 450-550 MPa, the yield strength Rp1,0 500-600 MPa and the tensile strength Rm 750-850 MPa after the heat treatment on the temperature range of 1000-1100° C.
In another preferred embodiment (B) the duplex stainless steel with the TRIP effect in accordance with the invention contains in weight % less than 0.04 carbon (C), less than 0.7% silicon (Si), less than 2.5 weight % manganese (Mn), 18.5-22.5% chromium (Cr), 0.8-4.5% nickel (Ni), 0.6-1.4% molybdenum (Mo), less than 1% copper (Cu), 0.10-0.24% nitrogen (N), optionally one or more added elements: less than 0.04% aluminium (Al), preferably less than 0.03% aluminium (Al), less than 0.003% boron (B), less than 0.003% calcium (Ca), less than 0.1% cerium (Ce), up to 1% cobalt (Co), up to 0.5% tungsten (W), up to 0.1% niobium (Nb), up to 0.1% titanium (Ti), up to 0.2% vanadium (V), the rest being iron (Fe) and inevitable impurities occurring in stainless steels. This duplex stainless steel is known from the WO patent application 2013/034804.
The duplex stainless steel of the embodiment (B) has the yield strength Rp0,2 500-550 MPa, the yield strength Rp1,0 550-600 MPa and the tensile strength Rm 750-800 MPa after the heat treatment on the temperature range of 950-1150° C.
The deforming of the ferritic austenitic duplex stainless steel according to the invention can be carried out by cold forming such as temper rolling, tension levelling, roller levelling, drawing or any other method which can be used for a desired reduction in a dimension or in dimensions of the object made of the ferritic austenitic duplex stainless steel.